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Biomedical subjects

M V Sefton

Publications and source records attributed to M V Sefton.

At least 73 records · Page 4Linked to original sources
PubMed1989

Microencapsulation of human diploid fibroblasts in cationic polyacrylates.

Human diploid fibroblasts and Chinese hamster ovary cells were encapsulated in several copolymers of dimethylaminoethyl methacrylate with methacrylic acid and/or methyl methacrylate. Copolymers containing 16 to 25% dimethylaminoethyl methacrylate and less than or equal to 2.2% methacrylic acid (based on monomer mol%) supported human diploid fibroblast growth when the polymer was cast as a film on glass or polystyrene. The cells survived encapsulation and grew, but growth was only observed in those capsules which appeared to be flawed; the flaws were detected as an early loss of fluorescence, due to leakage of the FITC-dextran added as a marker to the encapsulated cell suspension. Presumably the capsule wall had too low a permeability to allow for unrestricted growth. Chinese hamster ovary cells behaved similarly in dimethylaminoethyl methacrylate/methyl methacrylate capsules. Increasing the water content, by addition of methacrylic acid, did not improve matters, since these materials were not as good a substrate for cell growth as the others. Preparing materials that are sufficiently permeable, with low toxicity and high processability and which support the growth of anchorage-dependent cells is difficult, yet it remains an appropriate goal for further study.

Acrylates↗
PubMed1989

Inactivation of thrombin in heparin-PVA coated tubes.

Heparin, immobilized to polyvinyl alcohol by reaction with glutaraldehyde (heparin-PVA), retained its ability to accelerate the antithrombin III inactivation of thrombin, in a recirculating flow loop using heparin-PVA coated polyethylene tubes. The extent of inactivation, for a constant flow time, was approximately constant over ten cycles of exposure to thrombin and antithrombin III, suggesting that the immobilized heparin was reusable, as expected from the catalytic nature of non-immobilized heparin. Assessment of the chromogenic substrate activity of adsorbed thrombin and the extent of displacement were less conclusive with the implication that thrombin is adsorbed to heparin-PVA or PVA without heparin in multiple states.

Adsorption↗
PubMed1988

Permeability of a heparin-polyvinyl alcohol hydrogel to thrombin and antithrombin III.

The diffusivities of thrombin and antithrombin III in a heparin-polyvinyl alcohol hydrogel were estimated and used to demonstrate that diffusion limits the effectiveness of the immobilized heparin in the interior of such hydrogels. Diffusivities were calculated from permeabilities and partition coefficients measured with films in a diffusion chamber apparatus. The diffusion coefficients were estimated to be 6 +/- 4 X 10(-8) cm2/s for thrombin and 4 +/- 2 X 10(-8) cm2/s for antithrombin III in 10% gel membranes with or without immobilized heparin. Using the diffusivity of thrombin and a Thiele-type modulus, the effectiveness factor of a spherical heparin-PVA bead used to accelerate the inactivation of thrombin by antithrombin III was found to be 4-9% (diameter range 250-150 micron). While indicating that diffusion of thrombin limited the full utilization of the immobilized heparin, these values for the effectiveness factor could not completely account for the low apparent heparin activity (0.2%) in a thrombin time test of heparin-PVA "beads" (J. Biomed. Mater. Res., 17, 359 (1983]. Other factors such as the immobilization chemistry or the diffusion of thrombin-antithrombin III complex must be considered for a full explanation of the thrombin time results.

Animals↗
PubMed1988

Immobilization of prostaglandin PGF2 alpha on poly(vinyl alcohol).

Prostaglandin PGF2 alpha was immobilized onto a poly(vinyl alcohol) hydrogel (PVA) by reaction of a PGF2 alpha-polylysine adduct with glutaraldehyde. The PGF2 alpha-polylysine adduct was prepared by carbodiimide activation of the carboxyl group of PGF2 alpha followed by coupling to the lysine. The adduct was separated from the unreacted PGF2 alpha by dialysis and the purified product was found to contain congruent to 44 mol of PGF2 alpha/mol of adduct involving congruent to 40% of the amines of the polylysine. The adduct was bound to PVA by reacting with an excess of glutaraldehyde at 0 degrees C, followed by cross-linking of the PVA to a gel at 35-37 degrees C. The PGF2 alpha of the adduct was found to retain congruent to 40% of its biological activity on a molar basis in a smooth muscle cell contraction assay, but its activity immobilized to PVA was not determined. Spectroscopically, infrared nuclear magnetic resonance (IR/NMR), the PGF2 alpha appeared identical to the native molecule, except for the amide bond at its carboxylic acid, suggesting that the reactions were very gentle and that other biomolecules could be incorporated into the gel without loss of activity.

Amines↗
PubMed1988

Water content and compression modulus of some heparin-PVA hydrogels.

The effects of changes in the heparin-PVA formulation on the water content, compression modulus and molecular weight between cross-links of the resulting gel was determined. Molecular weight between cross-links was calculated directly from the compression modulus and the swelling ratio. Glutaraldehyde and MgCl2 displayed the effects expected of them as cross-linking agent and catalyst respectively. On the other hand, formaldehyde appeared to be a non-essential component since its absence had little effect on gel properties and its presence did not affect the water content as originally predicted. Understanding the effect of each formulation component on gel properties enables alteration of gel water content and hence permeability for special applications.

Biocompatible Materials↗
PubMed1988

Microencapsulation of mammalian cells in a hydroxyethyl methacrylate-methyl methacrylate copolymer: preliminary development.

Erythrocytes were microencapsulated in a thermoplastic copolymer of poly-2-hydroxyethyl methacrylate (79% mole%) - co-methyl methacrylate (21 mole %) with little apparent initial cell lysis. Droplets of cell suspension and polymer solution were blown from the tip of a coaxial needle assembly into a receiving bath of hexadecane over phosphate buffered saline (PBS) containing a low concentration of nonionic surfactant. Capsules were trapped at the hexadecane/PBS interface where they were cured by the removal of polymer solvent to precipitate a polymer coating around the cell suspension. Important principles which were considered in the development of the successful process, included the need to prevent intermixing of polymer solution and cell suspension, to fully surround the cells with polymer solution prior to precipitation, and to prevent direct mixing of the curing bath with the polymer solution.

Artificial Organs↗
PubMed1987

Microencapsulation of CHO cells in a hydroxyethyl methacrylate-methyl methacrylate copolymer.

Chinese hamster ovary fibroblasts, as model cells, have been microencapsulated in a hydroxyethyl methacrylate-methyl methacrylate copolymer (HEMA-MMA) by interfacial precipitation. The polymer containing approximately equal to 75 mol% HEMA, dissolved in polyethylene glycol 200 (PEG 200) was coextruded with the cell suspension (4-6 X 10(5) cells/ml in the alpha-MEM with 10% foetal calf serum +/- Ficoll 400/PBS) through a concentric needle assembly. Polymer solution droplets, containing cells, were blown off the end of the needle assembly by a coaxial filtered air stream into a nonsolvent bath containing phosphate buffered saline (PBS) with 5 ppm Pluronic L101, overlaid with hexadecane. The nascent capsules hang at the hexadecane/PBS interface while the solvent is extracted into the aqueous nonsolvent, to precipitate the polymer around the cells. The resultant capsules were 500 microns-1 mm in diam. with a microporous sponge-like interior, and also very tough and flexible. The cells survived encapsulation based on subculture ability, retention of some fluorescein diacetate (FDA) activity over 5 d and direct light microscopic evidence of cell growth over 10 d after histological sectioning and staining. However, cell growth was not uniformly observed (especially in the FDA assay) and this was attributed to space limitations for growth within the microporous interior. Continued development of this process and adaptation to cells such as pancreatic islets is expected to lead to hybrid artificial organs which are capable of ameliorating metabolic disorders such as diabetes.

Acrylates↗
PubMed1987

Graft copolymer emulsions of sodium alginate with hydroxyalkyl methacrylates for microencapsulation.

A hybrid material with interesting capsule forming properties has been prepared by gamma-initiated polymerization of hydroxyethyl methacrylate (HEMA) or other acrylic monomers in the presence of aqueous sodium alginate. For example, an aqueous solution of HEMA (1 wt%) and sodium alginate (1 wt%) was irradiated in a gamma-source at a dose of 0.15 MR, to produce a stable emulsion which coalesced in the form of a homogeneous precipitate when added to 0.1 M CaCl2. Polymerization of HEMA was presumed to be initiated by free radicals generated in the alginate by gamma-irradiation. The emulsion consisted largely of polyHEMA homopolymer stabilized by an alginate-polyacrylate graft (or block) copolymer of highly branched and ill-defined character, which acts as surfactant to prevent coagulation of the emulsion. Erythrocytes were encapsulated in such an emulsion by extrusion of cell/copolymer emulsion mixture into HEPES buffered CaCl2 (10 mM Ca). Cells appeared intact and functional after encapsulation. By SEM it appeared that the capsule wall consisted of microspheres of polyHEMA on the inner and outer surfaces held together by the alginate. Furthermore the capsule did not contain the internal meshwork, characteristic of calcium alginate homopolymer gels. Although much remains to be learned about these materials, the combination of the easy gelling characteristics of the alginate with the range of desirable properties (e.g. biocompatibility) afforded by the acrylic monomers may have a significant impact on the development of cell microencapsulation technology.

Alginates↗
PubMed1987

Implantable pumps.

Implantable drug-delivery pumps are being developed to provide the external control of delivery rate or to deliver volumes of drug that are beyond the capabilities of conventional controlled-release formulations. The delivery of insulin to diabetics and chemotherapeutic agents to cancer patients represents the two major applications of such devices, but other applications (chronic pain control, Alzheimer's disease, spasticity, etc.) exist or have been proposed. The most popular device is the Infusaid which is driven by a fluorocarbon vapor-liquid mixture to provide a constant delivery rate. Implantable peristaltic pumps have recently been developed to deliver drugs at variable rates according to a physician-preprogrammed schedule which is actuated with the aid of transcutaneous telemetry. Other devices are available in the literature, if not yet in clinical application.

Antineoplastic Agents↗
PubMed1986

Factor Xa inactivation by a heparinized hydrogel.

Incubating Factor Xa and plasma in the presence of "beads" of a heparinized hydrogel (heparin-PVA) resulted in a loss of Factor Xa activity as measured by clotting time or chromogenic substrate assays. This loss in activity was attributed to the activity of the heparin immobilized to the polyvinyl alcohol (PVA) hydrogel. The immobilized heparin retained an apparent activity approximately 1% that of a similar mass of heparin in solution. The loss in apparent activity was in part attributed to diffusion effects which limited the accessibility of the enzyme and inhibitor to the heparin immobilized in the interior of the gel "beads". The chromogenic substrate activity of adsorbed Factor Xa was also measured and was found to decrease after exposure to antithrombin III to similar extents regardless of the sequence of exposure. This suggests that Factor Xa and antithrombin III have similar affinities for this immobilized heparin, unlike the situation for thrombin (Thromb-Res., 20, 543-554, 1980).

Factor X↗
PubMed1986

A model of insulin delivery by a controlled release micropump.

A model has been developed to describe the delivery of insulin from a controlled release micropump (CRM). Basal delivery was provided by diffusion due to a concentration difference driving force across the CRM. This was modelled by considering the CRM to be a series of one-dimensional steady-state diffusion resistances. This delivery model was used to size prototypes and identify the piston, foam and the pump outlet as the controlling resistances to basal insulin transport. Augmented delivery by the CRM was achieved by repeated compression of a foam disk by a mild steel piston which was driven by a solenoid (tested voltage range 0-173 V DC; 5 msec "on" time; frequency 20-40 min-1). The increased delivery was attributed to the combination of mixing inside the pump barrel and displacement of barrel contents into the downstream reservoir. This action was approximated by a three-compartment model, which considered the CRM to consist of a well-mixed upstream reservoir and pump barrel (with a downstream reservoir) separated by two resistances: a constant upstream membrane resistance, (KmAm)-1, and a variable downstream mixing rate resistance, (Qd)-1. A least squares fit of the model to experimental data showed Qd to increase with the cube of the force on the piston and linearly with the compression frequency. In agreement with experimental results, the model predicted the upstream membrane to be rate controlling only at augmented pump resistances close to the value (KmAm)-1. These models were used to design an improved prototype (VIII) which is now being evaluated in vivo in pancreatectomized dogs for its efficacy in restoring and sustaining normoglycemia.

Animals↗
PubMed1986

Coating of two polyether-polyurethanes and polyethylene with a heparin-poly-(vinyl alcohol) hydrogel.

Two polyether-polyurethane elastomers (Pellethane and Biomer) and polyethylene were coated with a heparin-poly(vinyl alcohol) hydrogel. The requisite surface modification in preparation for coating consisted of glow discharge cleaning and acid treatment for the polyether-polyurethanes and glow discharge cleaning and chromic acid oxidation for polyethylene. The chemical modifications increased surface wettability. Surface analysis by attenuated total reflectance Fourier transform infrared spectroscopy indicated that the acid treatment caused hydrolysis of the polyether segments of Pellethane and Biomer. Prolonged partial thromboplastin times were observed on the coated films. The results of toluidine blue assay of heparin in the solution in which the coated films were immersed for a long time suggested that heparin was covalently bound in the coating. Such coating techniques extend the usefulness of the heparin-poly(vinyl alcohol) hydrogel to a number of medically important substrate materials.

Biocompatible Materials↗
PubMed1985

Parallel flow arteriovenous shunt for the ex vivo evaluation of heparinized materials.

The patency of heparin-polyvinyl alcohol (hep-PVA) coated polyethylene tubing was found to be significantly longer than control tubes coated with polyvinyl alcohol (PVA) but without heparin at low flow rates in dogs using a novel parallel flow arteriovenous shunt designed to avoid surgical artifacts. A standard Silastic chronic shunt (3.18 mm i.d.) was inserted between the iliac artery and vein of a dog. After a 2-week recovery period, a small diameter coated polyethylene tube (1.14 mm i.d.) was connected in parallel with the exteriorized portion of the chronic shunt through a pair of Silastic Y-connectors, so that less than 3% of the shunt flow was diverted into the test tube. The chronic shunt was reused many times over a greater than 6 month patency period, eliminating the need for frequent surgery and reducing interanimal variability in the results. The difference in patency between heparinized and control tubes was greater at higher mainshunt flow rates indicating the presence of a significant effect of the Y-connectors on platelet adhesion or aggregation. This effect was manifested in a time-dependent reduction in circulating platelet count. Scanning electron microscopy (SEM) examination of the midportion of the heparinized tubes after occlusion demonstrated the absence of platelet and fibrin deposits, unlike the control tubes without heparin. Although the Y-connectors played a significant role, they did not dominate the thrombotic processes occurring in this shunt and consequently the biological effectiveness of the immobilized heparin could be demonstrated.

Animals↗
PubMed1985

Laboratory evaluation of stain-length passive dosimeters for monitoring of vinyl chloride and ethylene oxide.

The effects of vapor concentration in the range of 1.2 to 5.1 ppm (vinyl chloride) and 8.3 to 29.1 ppm (ethylene oxide) on the response of new stain-length passive dosimeters were evaluated separately in a dynamic exposure chamber. The vinyl chloride dosimeter was prepared with a permanganate impregnated blend of Chromosorb W and silica gel, while a silica gel-coated plastic strip (TLC plate) impregnated with dichromate was used to detect ethylene oxide. The use of a TLC plate as the inert support allowed us to reduce the amount of reagent loaded per length of tube, thus significantly enhancing in the sensitivity of the unit, which was necessary for accurately detecting ethylene oxide at these low concentrations. At the vinyl chloride exposure of 8 ppm-hrs the length of stain was 0.76 cm and the 95% confidence interval about this point was +/- 1.4 ppm-hrs (18%). For the ethylene oxide dosimeter the length of stain at the exposure of 80 ppm-hrs was 0.90 cm and the 95% confidence interval about this point was +/- 16 ppm-hrs (20%). Although some shortening of the stain was noted at low relative humidity (26%) in the vinyl chloride device, no effect on the dosimeter response was observed over the range of relative humidity of 35 to 96%. The ethylene oxide dosimeter response was not affected by relative humidity in the range of 28 to 90%. The use of a TLC plate as the inert support of the colorimetric reagent has proven to be an excellent means of improving the sensitivity of these stain-length passive dosimeters.(ABSTRACT TRUNCATED AT 250 WORDS)

Air Pollutants, Occupational↗
PubMed1984

Ethylene-vinyl acetate copolymer microspheres for controlled release of macromolecules.

A simple technique has been devised for preparing ethylene-vinyl acetate copolymer microspheres containing macromolecular drugs. Beads with good sphericity were formed by a simple extrusion process that can easily be repeated by any laboratory without special equipment. The extruded droplets gelled on immediate contact with cold ethanol in a dry ice-ethanol bath. The ethanol also served to remove the solvent and harden the microspheres as they warmed to room temperature. Release of bovine serum albumin followed the expected trends, i.e., initial linearity with the square root of time and faster release rates with greater drug loading.

Chemistry, Pharmaceutical↗
PubMed1984

Adsorption isotherms of insulin onto various materials.

Insulin adsorption isotherms on various materials have been measured to begin to clarify the role of material surface energy in insulin aggregation. Using 125I-insulin and direct gamma-counting of the exposed material, more insulin (per unit area) was adsorbed to hydrophobic materials (Teflon, Silastic) than to the hydrophilic ones (polyacrylamide, glass) from insulin solutions ranging from 0.1 to 100 U/ml. For example, after 30 min at room temperature, Teflon disks adsorbed 4.0 X 10(4) microU/cm2 and glass beads adsorbed 3.0 X 10(3) microU/cm2 from a phosphate-buffered, 100-U/ml solution (pH 7.4). The Teflon value exceeded, by a factor of six, an estimated plateau surface concentration based on the molecule area (750 A2), suggesting the occurrence of multilayer adsorption. Changing the buffer to acetate and lowering the pH to 3.5 resulted in an increased surface concentration, while the addition of glutamic or aspartic acid at pH 3.5 reduced the surface concentration to be comparable with that observed on glass and less than that observed on Teflon from phosphate buffer. Increasing the temperature to 37 degrees C resulted in a small decrease in adsorption, consistent with the exothermic adsorption of other proteins on similar materials. However, the significance of the adsorbed molecules in nucleating aggregate formation relative to other contributing factors remains to be assessed.

Acrylic Resins↗
PubMed1983

Fate of thrombin and thrombin-antithrombin-III complex adsorbed to a heparinized biomaterial: analysis of the enzyme-inhibitor complexes displaced by plasma.

Heparin covalently-linked to polyvinyl alcohol (PVA) is a biomaterial which is of potential value as a non-thrombogenic coating. 125I-labelled thrombin adsorbed to heparin-PVA beads was not dislodged by phosphate-buffered saline, pH 7.4, although radioactivity was progressively displaced from the adsorbent by fibrinogen-free human plasma. Analysis by gel filtration and affinity chromatography showed that the released radioactivity was distributed between (thrombin-antithrombin-III) complex (approx. 70%) and, probably, (thrombin-alpha-2-macroglobulin) complex (approx. 30%). Less efficient thrombin displacement was obtained by either bovine serum albumin (5% w/v) or antithrombin-III-free human plasma: in the latter case, the dislodged enzyme was presumably associated with alpha-2-macroglobulin. Purified alpha-2-macroglobulin did not displace thrombin from heparin-PVA. The quantity of thrombin displaced by an alpha-2-macroglobulin-free plasma fraction compared well with fibrinogen-free plasma: The eluted enzyme was largely associated with antithrombin-III. Purified antithrombin-III did not displace thrombin from heparin-PVA despite causing greater than 70% inactivation of the bound enzyme. Subsequent treatment with fibrinogen-free plasma dislodged (thrombin-antithrombin-III) at a similar rate to that of bound thrombin. We conclude that plasma contains a component(s) which displaces (thrombin-antithrombin-III) complex from immobilised heparin: presumably this leaves the heparin sites free for further use in enzyme inactivation.

Antithrombin III↗